1. Field of the Invention
The present invention relates to high-speed rotary atomisers for applying powder coating. Such atomisers comprise a housing; a rotatable bell disc arranged at the front of the housing; a motor driving the bell disc accommodated in the housing; at least one powder supply channel passing through the housing and emerging at the front of the housing; and at least one high-voltage electrode arranged in the housing. The powder particles flowing through the powder supply channel are guided past the at least one high-voltage electrode for ionisation.
2. Background Art
High-speed rotary atomisers with what is called xe2x80x9cinternal chargingxe2x80x9d have been increasingly used in recent times. xe2x80x9cInternal chargingxe2x80x9d means that the high-voltage electrode, with which the powder coating particles are ionised, is located inside the housing of the high-speed rotary atomiser. xe2x80x9cInternal chargingxe2x80x9d is in contrast to xe2x80x9cexternal chargingxe2x80x9d, wherein the high-voltage electrode is generally provided outside the housing, in the form of a ring that surrounds the bell disc. With xe2x80x9cexternal chargingxe2x80x9d the air surrounding the external electrode is first ionised and then the coating powder is indirectly ionised via the air. Such a process has relatively low efficiency. With xe2x80x9cinternal chargingxe2x80x9d, by contrast, in an xe2x80x9cinternal chargingxe2x80x9d system, the coating powder particles are ionised by direct contact with the high-voltage electrode This process takes place with greater efficiency.
With known high-speed rotary atomisers of the kind mentioned at the outset, the coating powder particles flow through the powder supply channel at substantially constant velocity. As this happens coating powder particles are deposited on the walls of the powder supply channel, forming a substantially continuous, thin layer. As long as the coating powder particles have good electrical insulation properties this does not present problems. However, coating powders to which metallic particles are admixed to achieve certain optical effects are being increasingly used. If these metallic particles are deposited on the walls of the powder supply channel in the coating powder layer mentioned, this can lead to a backflash of high voltage from the high-voltage electrode towards the coating powder supply, which it is essential to prevent.
An object of the present invention is to configure a high-speed rotary atomiser of the type mentioned at the outset, in a simple manner, such that there is no danger of backflash from the high-voltage electrode even when using electrically conductive particles in the powder coating.
In the present invention, the powder supply channel contains an acceleration jet in which the coating powder particles are accelerated to a higher velocity which is maintained over at least a certain distance.
According to the invention it is ensured that the stream carrying the coating powder particles is accelerated in a specific section of the powder supply channel to such high velocity that no particles, and in particular no electrically conductive particles, can be deposited on the walls of the powder supply channel at least in that section. The high velocity, and therefore the cleanliness of the walls of the powder supply channel, must be maintained over such a distance that no backflash across this distance is possible with the high voltages used. In the portion of the powder supply channel located further downstream of the acceleration jet the stream of coating powder particles can decelerate again. If a layer of coating powder particles is deposited there on the surfaces of the powder supply channel, that is not hazardous.
The embodiment in which the acceleration jet has an Inlet through which pressurised acceleration air can be admitted to the passage of the acceleration jet is especially preferred. This additional acceleration air has two effects: firstly, it increases the acceleration of the stream of powder particles; secondly, with suitable aerodynamic guidance it produces an air layer between the wall of the powder supply channel and the coating powder stream located radially further inwards. This also contributes to keeping the coating powder particles away from the wall of the powder supply channel.
A further advantageous feature is the geometric configuration of the acceleration jet, by which the inlet has an annular groove in the jacket surface, an annular groove in the end face on the inlet side and at least one axial bore in the jet body of the acceleration jet connecting the annular grooves. In this case the acceleration air flows from the annular groove in the end face along the end face of the jet body into its passage. By this means a uniform inflow of acceleration air across the whole area of the passage of the acceleration jet is ensured.
In this embodiment the inlet for the acceleration air can include a narrow, radial gap leading to the inlet side of the passage of the acceleration jet. This gap is delimited on one side by the body of the acceleration jet and on the other side by a second, adjacent component. It is easily accessible for cleaning if needed.
An appropriate configuration of the high-speed rotary atomiser according to the invention is such that the acceleration jet passage first narrows in the flow direction towards a narrowest point and immediately widens again. In this case special external auxiliary means for accelerating the coating powder stream are not in principle required, since the flow restriction alone generates the desired increase in velocity. However, this does not preclude the use of additional acceleration air as mentioned above.
If acceleration air supplied along the end face of the acceleration jet body is used, it is recommended that the acceleration jet passage has a transitional section, formed by a plurality of juxtaposed conical annular surfaces with differing cone angles, extending from the inlet side to the narrowest point. It has emerged that this juxtaposing of conical faces causes less detachment and turbulence of the acceleration air than would be the case with a continuously curved transitional section.
It is also advantageous if the acceleration jet passage widens conically in the flow direction after the narrowest point. The conical widening must take place so slowly that the desired higher velocity is maintained over a sufficient distance. On the other hand, the widening must be so great that after flowing through the narrowest point of the acceleration jet the coating powder stream can be stabilised so that the least possible turbulence arises.